Electrostatic painting of substrates is desirable because it can reduce paint waste and emissions as compared to non-electrostatic painting techniques. Electrostatic painting techniques require the substrate to be electrically conducting or to have an applied prepcoat or primer which is electrically conducting, however, in order to display an increased paint transfer efficiency. Currently, injection molded articles are generally made from thermoplastic materials like Xenoy or TPO which are electrically insulating. This quality of the material makes electrostatic painting of articles molded from such resins more complex since an electrically conducing primer must be applied to the article prior to painting using this technique. Transfer efficiency using non-electrostatic techniques is less than 30% on Xenoy itself, whereas Xenoy having a coating of a conducting primer has a transfer efficiency during electrostatic painting approaching 55%. When using an electrically conducting primer, the path to ground is achieved via the conducting primer. This pathway in Xenoy articles is not at all well defined and is attributed to wrap of the primer around the rear of a supported part to a grounded buck. An alternative technique is to use a grounding clip. This undesirably causes higher film builds near the grounding clip with film builds decreasing as the distance from the grounding clip increases. In addition, after several passes through the paint booth, significant resistance to ground may be encountered due to multiple paint layers on the buck itself.
It would be desirable if the electrical conductivity of the substrate bulk material itself could be increased. This would overcome problems associated with electrostatically painting articles whose electrical conductivity is provided only through a conductive primer or prep coat. One solution might be to incorporate additives into the thermoplastic polymer to increase its bulk conductivity. Additives which might be incorporable in other types of polymer, e.g., stainless steel fibers in polyolefin polymers, are not suitable in thermoplastic polymer blends such as TPO or Xenoy because of a degradation of their desirable mechanical properties such as impact strength and tensile elongation, as well as a significant increase in the brittleness of the modified polymer blend. The present invention overcomes these problems.